Fluid flowmeters of the pressure drop or differential pressure type are well known. Orifice plate and venturi flowmeters are probably the most common of the pressure drop type. Orifice plate flowmeters are inexpensive, but they are inherently high energy loss devices since the measured pressure drop across the orifice is non-recoverable, i.e., the drop in pressure is a drop in total pressure. Venturi flowmeters are low energy loss devices relative to orifice plate flowmeters since most of the pressure drop in the venturi throat is recoverable at the venturi outlet, i.e., the drop in pressure is due to an increase in kinetic energy of the fluid. However, when either of these flowmeters are used to measure fluid flow which varies over a wide range, such as airflow to an automotive engine, they either overly restrict total airflow at high engine speeds and loads if they are sized small enough to provide an adequate differential pressure signal at low engine speeds and loads, or they provide an inadequate differential pressure signal at low engine speeds and loads if they are sized larger.
As government mandated fuel economy and exhaust emission standards become more stringent, the need for an inexpensive but accurate fluid flowmeter is much more acute. Simplicity of design is desirable not only from a cost standpoint but also for long term performance predictability. In the environment of a modern automobile engine, prior art designs employing moving parts often prove unreliable over extended periods of time as fatigue, corrosion and contaminates adversely effect operation. Prior art designs which have minimized or eliminated moving parts such as ion drift, hot wire anemometer and vibrating reed types, typically are not commercially feasible in automotive applications. In addition, such devices are designed for use with an engine of a predetermined displacement and fine tuning the flowmeter for use with a specific engine or recalibrating the flowmeter for use with an engine having a different displacement and flow characteristic is often impractical.